The present invention relates to the art of wireless telecommunication networks. It finds particular application in conjunction with third generation (3G) wireless systems using code division multiple access (CDMA) technology, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Walsh codes, spreading codes, channelization codes and the like are generally known in the art of wireless telecommunication networks. In particular, Walsh codes and/or Walsh functions are based on the Walsh-Hadamard matrices. However, for simplicity herein, the terms Walsh code and/or Walsh function are used to refer generally to any similarly employed spreading codes/functions, channelization codes/functions, etc. In CDMA, Walsh functions are used in a forward direction to organize network traffic over an air interface into different channels that can be isolated and decoded by target mobiles, e.g., wireless telephones, wireless personal digital assistants (PDAs) or other wireless devices. The forward or downlink direction refers to a transmitting direction from a base station to a mobile station.
At any given time for the same sector/carrier within a given cell site of a wireless telecommunications network, all the Walsh codes in use have to be mutually orthogonal with each other in order to properly organize the network traffic without interference or cross-talk between the different channels. This restriction was not particularly problematic for second generation wireless systems using CDMA. Second generation wireless generally encompasses the so called digital personal communications service (PCS). In any event, 2nd generation systems using CDMA only employ Walsh codes of a single size or bit length and all the codes used are guaranteed to be orthogonal to one another. For example, 64 Walsh codes each 64 bits in length are used in the typical implementation of 2nd generation systems.
As opposed to the 2nd generation, 3G wireless systems employing CDMA use Walsh codes of varying sizes or bit lengths. For example, traffic such as voice calls typically continue to use 64-bit Walsh codes. However, in 3G, some voice calls or traffic may use 128-bit Walsh codes. Similarly, for high-speed data traffic (e.g., wireless Internet access), 3G wireless makes available a variety of Walsh codes with shorter lengths, e.g., 32, 16, 8 and 4 bit lengths. Accordingly, unlike the 2nd generation which uses uniformly sized Walsh codes, Walsh code allocation in 3G CDMA wireless is not trivial. Walsh code allocation refers to the selection and/or assignment of Walsh codes for the different channels of cell traffic. Generally, it is preferable to employ shorter bit length Walsh codes for higher speed traffic.
In 3G CDMA wireless, variable size Walsh codes are made available for use. Consequently, all the available Walsh codes for the same sector/carrier within a given cell site are not guaranteed to be mutually orthogonal and their allocation fails to be a trivial matter. More specifically, each Walsh function in the set of Walsh functions having a bit length or size of N is non-orthogonal to: two Walsh functions in the set of Walsh functions of size 2N; four Walsh functions in the set of Walsh functions of size 4N; eight Walsh functions in the set of Walsh functions of size 8N; and so on. In particular, WkN is non-orthogonal to:                Wk2n and W(k+N)2n;        Wk4N, W(k+N)4N, W(k+2N)4N and W(k+3N)4N;        Wk8N, W(k+N)8N, W(k+2N)8N, W(k+3N)8N, W(k+4N8N, W(k+5N)8N, W(k+6N8N and W(k+7N)8N;        . . .        W(k)(2^n)N, W(k+N)2^n)N, W(k+2N)(2^n)N, W(k+3N)(2^n)N, W(k+4N)2^n)N . . . and W(k+((2^n)−1)N)(2^n)N.Regarding notation, WN represents the set of Walsh functions/codes having a size or bit length of N, and WkN represents the kth element of WN (note: the number or value of k used to reference a particular element does not necessarily equate to the binary representation of the corresponding Walsh code).        
Accordingly, the problem presented with the advent of 3G CDMA wireless involves the manner in which to allocate Walsh codes while ensuring the mutual orthogonality of all the concurrently used codes for the same sector/carrier within a given cell site. It is desirable, moreover, to carry out the allocation in such a manner that the number of Walsh codes remaining available for allocation at any given time is maximized. In this manner, efficient use of the cell's finite bandwidth and/or finite number of Walsh codes can be achieved. It is also desirable to maintain as great of a variety of Walsh code sizes available at any given time so that the greatest range of access speeds can be optimally accommodated (i.e., appropriately sized Walsh codes can be allocated) at any given time. Typically, this means reserving shorter bit length Walsh codes whenever possible.
Even though all the concurrently allocated Walsh codes may be mutually orthogonal, it is possible for the allocated Walsh codes to be selected such that the signal that is sent to the base station's amplifier exceeds an acceptable peak-to-average power ratio. The risk is that without a suitable allocation system the amplifier will be caused to operate in a non-linear manner and undesirable interference will be experienced in one or more frequency spectrums where it should not be. Accordingly, it is also desirable that the Walsh code allocation be implemented to avoid or minimize this risk, i.e., to maintain the peak-to-average power ratio at or below acceptable levels.
The present invention contemplates a new and improved Walsh code allocation/de-allocation system and method which overcomes or minimizes the above-referenced problems and others.